4.9 stumbling right off idle
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HT4100 4.1, 4.5, 4.9 Discussion, 4.9 stumbling right off idle in Cadillac Engine Technical Discussion; Hey, guys, just signed up on the forum. I have a '92 Eldorado with the 4.9, which has a slight ...
  1. #1
    seagull369 is offline Cadillac Owners Member
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    4.9 stumbling right off idle

    Hey, guys, just signed up on the forum.

    I have a '92 Eldorado with the 4.9, which has a slight problem. When I slightly depress the accelerator while in gear, the engine starts stumbling. Not to the point it'll stall, just runs real rough. At idle and under moderate to heavy acceleration, all is well- runs smooth with plenty of power.

    I noticed the check engine lite blip on once while driving, but it's not on anymore. Would like to check the stored code, but don't see a diagnostic connector under the hood to jump pins on, as I have in other OBDI cars. I heard a rumor pushing a couple of buttons on the air conditioning panel can get the light to start blinking its secrets, but the buttons on mine are all messed up so I can't do that.

    I just did a tune up on the thing, but of course the problem persists. Off the top of my head, I suspect either a TPS or MAP is to blame, but wanted to confirm that before I start plunking down the greenbacks for either.

    Any suggestions regarding what's up with my Eldo would be greatly appreciated. Thanks!

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    Sevillian273's Avatar
    Sevillian273 is offline Cadillac Owners Connoisseur
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    Re: 4.9 stumbling right off idle

    The ALDL connector is probably down by the emergency brake under the dash. Autozone will pull your codes for free. From what you describe, cam sensor fault comes to mind.... (E041) Just a guess.

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    seagull369 is offline Cadillac Owners Member
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    Re: 4.9 stumbling right off idle

    Hi and thanx for replying. I think Autozone only has a scanner for OBDII systems, last time I checked. The location you mentioned for the diagnostic port is also typically where the OBDII one is.

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    the recluse is offline Cadillac Owners Connoisseur
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    Re: 4.9 stumbling right off idle

    Read the "How to pull codes" section in the Eldorado/Seville forums and see if that helps.

    Could be failing MAP or TPS, that's what I'm leaning towards myself.

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    seagull369 is offline Cadillac Owners Member
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    Re: 4.9 stumbling right off idle

    Well, a new MAP didn't solve the problem. I tested the EGR valve and it's not holding vacuum. I removed and capped the vacuum line from it and drove the car with my fingers crossed, but the problem remains. I've heard if an EGR is stuck open it could cause the hesitating problem I've described. In any regards, it needs to be replaced and I'll report back any findings when I do that.

    I found the ECU port right where you said it was, Sevillian273, thank you. I jumped the pins as per Haynes' instructions, but apparently instead of morse-coding the check engine light, this car seems to actually write out in words what's up on a small digital display under the speedo (unless I did something wrong). The only thing that really came up under that was "set timing mode." Not sure what that means.

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    daniel58 is offline Cadillac Owners Fanatic
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    Re: 4.9 stumbling right off idle

    You really need to follow Recluse's instructions and read up on how to pull codes, I couldn't begin to explain it here. Its sounds like what you did was to disable the computer advance to set base timing.

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    seagull369 is offline Cadillac Owners Member
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    Re: 4.9 stumbling right off idle

    There is a problem with the buttons on the HVAC control, Daniel, so pulling up the codes as per that sticky does not work for me. The Haynes manual mentioned jumping those pins in order to retrieve the codes, but I think you're right that all it did was allow for base timing to be set.

    I got a new EGR but when I tested it, noticed that it didn't hold vacuum either. I doubt this one's bad too, so that makes me wonder if these EGR's aren't meant to work that way or something. Beats me. I'm about to give up on this thing.

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    83CADMAN's Avatar
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    Re: 4.9 stumbling right off idle

    Quote Originally Posted by seagull369 View Post
    I got a new EGR but when I tested it, noticed that it didn't hold vacuum either.
    Did you use a hand vacuum pump to check EGR?
    You may need to replace the length of hose to the EGR valve.
    Also the EGR control solenoid or regulator could be leaking.

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    83CADMAN's Avatar
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    Re: 4.9 stumbling right off idle

    Quote Originally Posted by seagull369 View Post
    There is a problem with the buttons on the HVAC control,
    Check out your local wrecking yards for a replacement. Plenty of 92's out there.

  11. #10
    daniel58 is offline Cadillac Owners Fanatic
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    Re: 4.9 stumbling right off idle

    It would be a good idea to check all of the vacuum hoses, and check for loose connectors if you haven't already done so. If you cannot use the onboard diagnostics you will have to check for codes with a scanner, all repair shops have them. I have a 4.5 that has the same problems as yours does with no codes and I have replaced everything (and I mean everything!) with no results. The guys on here are stumped as I am. See distrubutor 180 out post. If you ever do figure it out please let me know what the problem was. Good luck!

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    seagull369 is offline Cadillac Owners Member
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    Re: 4.9 stumbling right off idle

    After checking out the Haynes manual again, turns out the EGR valve gets tested while the engine is running and there's sufficient back pressure (they recommend clamping a large socket in the tailpipe to create that). Without back pressure it seems the EGR will leak vacuum just like it did when I tried to test it with engine off (and yea, I used a hand pump tester to do it). Once I test it that way and it checks out, I'll check the vacuum solenoid. All vacuum hoses in route are in good shape, btw.

    Haynes unfortunately does not describe a procedure for testing the TPS on the 4.9 (which was one of my hunches for cause of problem). It only mentions voltage varies proportionally with key on and throttle lever is moved. There are 3 wires going to the sensor and I might try stabbing a few with my multimeter to see what I come up with. In addition to that sensor, I was wondering if the IAT (intake air temp.) sensor could be a possible culprit as well. Haynes doesn't describe how to test that either .

    I'm going to the boneyard this week, will try to find another HVAC control. If I ever figure out what's causing the problem, I'll definitely let you know daniel58.

    Thank you all for your help thus far.

  13. #12
    the recluse is offline Cadillac Owners Connoisseur
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    Re: 4.9 stumbling right off idle

    All the testing and setup procedures (as well as pulling codes) are done through the HVAC controller, so by all means, get a new one if you need it. Reply back when you get it in and I'll give you the rundown on the procedures.

  14. #13
    drewsdeville is offline Cadillac Owners Master
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    Re: 4.9 stumbling right off idle

    If you don't get the display/buttons working, you can do many tests with a VOM but it is more of a hassle.

    If you want to test the TPS, backprobe the TPS connector using the blue wire and a good ground. As you increase throttle, the voltage should increase steadily with no flat spots.

  15. #14
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    cctgene is online now Cadillac Owners Fanatic
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    Re: 4.9 stumbling right off idle

    For what it’s worth, things to check for engine stumble from 92 FSM

  16. #15
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    hcaddy95 is offline Cadillac Owners Fanatic
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    Re: 4.9 stumbling right off idle

    So did you Check the timing, check the idle speed control motor to see if its adjusted correctly i think you could take your IAT sensor and clean it also and advance your timing alittle bit to like 12-14 degrees

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